Stacking Interactions between Square-Planar Metal Complexes with 2

Article pubs.acs.org/crystal

Stacking Interactions between Square-Planar Metal Complexes with 2,2′-Bipyridine Ligands. Analysis of Crystal Structures and Quantum Chemical Calculations Predrag V. Petrović,†,# Goran V. Janjić,‡,# and Snežana D. Zarić*,#,§,∥ †

Innovation Center, Department of Chemistry, Studentski trg 12-16, 11000 Belgrade, Serbia ICTM, University of Belgrade, Njegoševa 12, 11000 Belgrade, Serbia § Department of Chemistry, University of Belgrade, Studentski trg 12-16, 11000 Belgrade, Serbia ∥ Department of Chemistry, Texas A&M University at Qatar, P.O. Box 23874, Doha, Qatar ‡

ABSTRACT: Stacking interactions between square-planar metal complexes containing bipyridine ligands (bipy) were studied by analyzing data in the Cambridge Structural Database (CSD) and by density functional theory (DFT) calculations. In most of the crystal structures, two bipy complexes were head-to-tail oriented. On the basis of the data from CSD, we classified the overlaps of bipy complexes into six types. The types were defined by values of geometrical parameters, and the interactions of the same type have very similar overlap geometries. The most frequent are the structures with quite large overlap area including chelate rings and pyridine fragments. The overlap is often influenced by ligands coordinated at the third and fourth coordinating positions or by molecules (ions) from the environment in the crystal structure. The interaction energies of all types of overlap were calculated on model systems using the DFT (TPSS-D3) method. The strongest calculated interaction has an energy of −31.66 kcal/mol and large area of overlap. By decreasing the overlap area, the strength of interactions decreases. The weakest calculated interaction has an energy of −7.26 kcal/mol and the small overlap area of pyridine fragments. These results presenting the geometries and energies of stacking interactions can be very important for various molecular systems.

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INTRODUCTION

Planar chelate rings can also form stacking interactions with other chelate rings.59−61 We have found large number of chelate−chelate stacking interactions in the crystal structures from the CSD.62 The normal distances in chelate−chelate stacking interactions are similar to those found in stacking interactions between organic aromatic rings, while the of fset of two interacting chelate rings can be different62 than between organic aromatic rings. In our pursuit to better understanding of stacking interactions, we studied square-planar transition metal complexes with terpyridyl (terpy)63 and phenantroline (phen)64 ligands. We analyzed the data in the crystal structures from the CSD for stacking interactions of terpy and phen complexes. The results show that in stacking interactions both terpy and phen complexes have a preference for head-to-tail orientation. Bipyridine (bipy) complexes can be used for various applications. They can be used in photoinduced65−70 and electron-transfer processes66,69,71 or in supramolecular architectures and materials science.69,72,73 Unfunctionalized, many monofunctionalized,66,72 symmetrical,69 and unsymmetrical multifunctionalized bipyridines74 have been synthesized and used for dye-sensitized solar cells (DSC),65,68,70 functional

Noncovalent stacking interactions between aromatic rings are all-present in nature, but their accurate characterization still remains a challenging task, both theoretically and experimentally. In the last couple of years, new methods have been developed and used, particularly with stacking interactions in mind.1−10 Generally, stacking interactions between aromatic organic molecules or fragments were studied,11−34 but it was shown that other planar molecules and fragments could also form parallel interactions.35−48 By analyzing the data in the Cambridge Structural Database (CSD), stacking interactions between chelate and C6-aromatic rings were identified in the crystal structures of square-planar transition metal complexes.44−46 Planar chelate rings with delocalized π-bonds can also form CH/π interactions with C6-aromatic rings.49−57 However, analysis of the CSD has shown that stacking interactions are preferred to CH/π interactions in these complexes.43 The energies of stacking interactions of benzene with chelate ring were calculated,58 and results show that chelate−benzene stacking interactions are remarkably stronger than benzene−benzene interactions. The calculated stacking energies of copper and nickel chelates with benzene are −6.39 kcal/mol and −4.77 kcal/mol, respectively,58 while the calculated energy of benzene−benzene interaction is −2.73 kcal/mol.5 © 2014 American Chemical Society

Received: April 1, 2014 Revised: May 23, 2014 Published: May 29, 2014 3880

dx.doi.org/10.1021/cg500447h | Cryst. Growth Des. 2014, 14, 3880−3889

Crystal Growth & Design

Article

structures, (e) structures with disorder were not included, (f) structures solved from powder were not taken into account either. In order to find intermolecular stacking interactions between bipyridines, we conducted three searches. In the first search, we looked for the structures with the distance between centroids of any pyridine fragment (dPP distance, Figure 1) below 4.6 Å,83 in the second the criterion was that the distance between centers of pyridine and chelate rings (dPC distance, Figure 1) is shorter than 4.4 Å, and in the third the criterion was that distance between centroids of the chelates (dCC distance, Figure 1) is below 4.2 Å. Results from all three searches were combined and analyzed together in order to ensure that all possible stacking interactions are included. We considered that two rings form stacking interactions when the dihedral angle between the mean planes of bipyridines is less than 10°. The data related to the intermolecular stacking interaction were analyzed by the use of the structural parameters presented in Figure 1. The coordinated bipy ligand has three fused rings: two pyridine fragments and one chelate ring. Distances dPP and dCC are the shortest intermolecular distances between centroids of two pyridine fragments (ΩP1 and ΩP2) and two chelate rings (ΩC1 and ΩC2), respectively. The shortest distance from the pyridine ring center (ΩP1) of the first bipy ligand to the center of chelate ring of the second one (ΩC2) is referred to as dPC. The distance between the metal ion of the first complex (M1) and the projection of the metal ion of the second complex (M′2) onto the average plane of the first one represents the horizontal rMM displacement. The normal distance between the planes of the interacting rings is R. The angle between the M1−M′2 and M1−ΩC1 directions in the plane of the first complex is denoted φ, where the center of chelate ring is shown as ΩC1. Torsion angle T1 is the ΩC1− M1−M2−ΩC2 angle, while T2 is Ωp1−Ω′p1−Ω′p2−Ωp2 angle. Ωp1 and Ωp2 are the centers of pyridine rings in two complexes with the shortest dPP distance. To illustrate the geometry of stacking interactions and the effect of the supramolecular structure on stacking interactions of two bipy ligands, a few examples of crystal structures were presented. For this purpose, the structures with typical geometric parameters for a certain type of overlap (parameters rMM and φ) and structures with simple metal complexes were selected. DFT Calculations. In order to evaluate the energy of stacking interaction between two square-planar bipy complexes, the single-point calculations were done in Gaussian09 (version D.01)84 program, using the TPSS-D385,86 method and the def2-TZVP87 basis set. The calculations were performed on model systems of (2,2′-bipyridyl)dicyanonickel(II) complexes, made from crystal structures. Since the molecules in which these interactions occur are rather large, and there are at the same time other intermolecular interactions, the calculations were performed on smaller model systems. These model systems were built from the crystal structures by substituting all metal ions with Ni(II) ion and by substituting all other ligands with CN ligands. The geometries of the stacked bipy ligands were kept the same as they were in the crystal structures. The geometry of the two remaining ligands (CN ligands) were taken from the optimized structure of (2,2′bipyridyl)dicyanonickel(II) complex, obtained at TPSS-D3/SDD88/ def2-TZVP level of theory. The interaction energies between two bipy nickel(II) complexes were calculated at TPSS-D3/def2-TZVP level, and the basis set superposition error (BSSE) was removed by counterpoise (CP) correction.89 We chose TPSS dispersion corrected functional because it is one of the well-established methods for transition metal chemistry,90−92 It was shown that TPSS-D3/def2TZVP level is in excellent agreement with CCSD(T)/CBS values.58 Namely, calculated energy of stacking interaction between nickelchelate and benzene ring is −4.28 kcal/mol, at TPSS-D3/def2-TZVP level, while the value of energy evaluated at CCSD(T)/CBS level is −4.48 kcal/mol.58 Calculations at TPSS-D3/def2-TZVP level do not consume much time, which is appropriate for a large system calculated in this work.

polymers, new catalytic species, molecular recognition systems, etc.69,72 For example, [Ru(bipy)3]2+ has been used in various cases: from unique photoprobes of host−guest interfaces with enhanced potential for photonic and sensor applications with intense luminescence67 to usage in combined tiny films of modified electrodes which exhibit improved electron transfer, electrocatalytic and fluorescence activity.68,75,76 It was found that some bipy complexes can bind to DNA with high binding constants. It was also shown that the complexes can bind to DNA by intercalation that is proven by hypochromism and bathochromism of electronic absorption spectra or interact with the DNA surface by π−π stacking interactions.77−79 In order to better understand stacking interactions of bipy complexes, here we analyze the geometries of stacking interactions between bipy square-planar metal complexes in crystal structures from the CSD. The results of crystallographic analysis were accompanied by the results of DFT calculations performed on model systems. To the best of our knowledge, this is the first study on stacking geometries of bipy squareplanar metal complexes in crystal structures from the CSD and the first study reporting the energies of stacking interactions of bipy complexes.

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METHODOLOGY

The Search of CSD. CSD search (November 2012 release, version 5.3480) was performed using the ConQuest1.15 program81 to extract all structures containing square-planar bipy complexes. The search was based on the fragment defined in Figure 1. The queries for the crystal structure search were made according to the following criteria: (a) the crystallographic R factor < 10%, (b) the error-free coordinates according to the criteria used in the CSD, (c) the H atom positions were normalized using the CSD default X−H bond lengths (O−H = 0.983 Å; C−H = 1.083 Å; and N−H = 1.009 Å),82 (d) no polymer

Figure 1. Geometrical parameters used for describing the stacking interactions between bipy complexes. Figure shows orientations with T1 = 0.0° and 180° and T2 = 0.0° and 180°. 3881



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terpy and phen complexes,63,64 we also found preference for head-to-tail orientations. The distribution of T2 torsion angle also shows two preferred orientations: the first orientation with T2 values of 0° to 10° and the second one with 170° to 180° (Figure 3). A small number of the interactions have T2 values between 10° and 170°. The values of T2 torsion angle of 0−10° correspond to the interactions with overlap of a large part of bipy ligands, while the values of 170−180° correspond to only partial overlap of bipy ligands (Figure 1). The interactions with the values of T2 in the range from 0° to 10° occur more frequently. We obtained similar results for terpy complexes,63 while for phen complexes64 there is less pronounced tendency for orientation with T2 values of 170° to 180°. The reason for such behavior of phen complexes might be the existence of an additional aromatic ring in the phen ligand. Because of the small number of structures with head-to-head orientation (T1 values of 0° to 10°) (Figure 3), we analyzed only the structures with head-to-tail orientation (T1 values of 170° to 180°). Structures with head-to-tail orientation form two groups of contacts; in the first group of contacts (I) T2 torsion angle is close to 0°, while in the second group (II) torsion angle T2 is close to 180°. The first group of structures (I) includes 170 interactions. Histograms presenting the distribution of horizontal displacements rMM and normal distances R are shown in Figure 4. The histogram shows that most of the displacements rMM are distributed in the range of 3.0−7.0 Å, with the maximum between 4.0 and 6.0 Å. In most of these structures, the normal distances R between the planes of interacting bipy ligands are in the range of 3.3−3.7 Å, with maximum at 3.3−3.5 Å (Figure 4). In this first group of structures (I), with T2 torsion angle close to 0°, a large area of the bipy ligand can be involved in overlap (Figure 1). For a description of these interactions two geometrical parameters were used: horizontal displacement rMM and angle φ (Figure 1). In our previous work, we showed that using these two parameters enables the description of stacking interactions of terpy and phen complexes.63,64 The plot of angle φ versus the offset rMM for group I of bipy complexes is shown in Figure 5. In group I values of angle φ are less that 40°. The scattergram in Figure 5 for group I does not show clustering. However, the interactions in this group can be quite versatile, as indicated by the wide range of observed rMM values. Different types of overlap in group I are defined based on rings (chelate and pyridine) of bipy ligand involved in the overlap. At different values of rMM different rings overlap. In geometries with rMM values lower than 3.0 Å (overlap type Ia), only chelate rings of bipy ligands overlap (Figure 6). At larger rMM values, between 3.0 and 5.0 Å, chelate−chelate, chelate−pyridine, and

RESULTS AND DISCUSSION Geometries in Crystal Structures and Interaction Energies. By searching the crystal structures of square-planar bipy metal complexes in the CSD, 325 interactions were found in which two complexes with mutual parallel orientation are in close contact, with the distance between the centers of the two pyridine rings shorter than 4.6 Å, or the distance between centers of pyridine and chelate rings shorter than 4.4 Å, or the distance between centers of two chelate rings shorter than 4.2 Å. Mutual orientations of interacting complexes are described by geometrical parameters shown in Figure 1. In most of the structures the normal distances between the planes of interacting square-planar bipy complexes are in the range from 3.2 to 3.7 Å. The maximum of the distribution is at 3.3−3.5 Å (Figure 2). Similar normal distances were found for terpy and phen complexes.63,64

Figure 2. Histogram shows the distributions of the normal distances (R) for stacking interactions of square-planar bipy complexes.

To define the orientation of metal complexes, two torsion angles (T1 and T2) have been analyzed. Geometries with T1 angle values from 0° to 10° correspond to head-to-head orientation, while geometries with T1 angle values from 0° to 10° correspond to head-to-tail orientation (Figure 1). In geometries with T2 angle values from 170° to 180°, both pyridine rings of the bipy ligand participate in the stacking interaction, while in geometries with T2 angle values from 170° to 180° only one pyridine ring participates in the interaction (Figure 1). The distribution of T1 torsion angle values shows preferred orientation with the angle from 170° to 180° (head-to-tail orientation) (Figure 3). Only 18 interactions have head-to-head orientation (T1 values of 0° to 10°). The number of interactions with T1 between 10° and 170° is quite small. In our previous work on stacking interactions of square-planar

Figure 3. Histograms showing the distributions of torsion angles T1 and T2, for interactions of square-planar bipy complexes. 3882



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Figure 4. Distributions of rMM parameter and normal distance R (Å) for the interactions in group I.

6), which represents thiocyanato-(isothiocyanato)-(2,2′-bipyridine)-platinum(II), is an illustration of this overlap. For this

Figure 5. Plot of angle φ versus of fset rMM, for the interactions in group I.

Figure 6. Perspective view of crystal fragment of AREHOU (thiocyanato-(isothiocyanato)-(2,2′-bipyridine)-platinum(II)), as a selected example for structures of type Ia and view on model system of (2,2′-bipyridyl)dicyanonickel(II) complexes, made from this crystal structure.

pyridine−pyridine overlaps occur (overlap type Ib, Figure 7). At rMM values between 5.0 and 6.0 Å, pyridine−chelate and pyridine−pyridine overlaps occur (overlap type Ic, Figure 8), while in geometries with rMM values larger than 6.0 Å (overlap type Id), only pyridine−pyridine overlap occurs (Figure 9). Data in Table 1 show geometrical parameters for all overlap types. The data in Figure 5 seems to be scattered along a trendline with negative slope. Such a trend is the consequence of the geometric constrains for group I. All interactions in group I have T2 torsion angle values close to 0°, and it means that with increasing rMM angle φ decreases. Namely, the structures with large rMM and large angle φ have T2 torsion angle close to 180° and belong to group II (Table 1). The structures with overlap type Ia include 14 interactions (Table 1). Bipy ligands are oriented in such way that overlap occurs only between chelate rings. In most of the structures other ligands coordinated to a metal are planar. In these structures pyridine rings of bipy overlap with other ligands from the other interacting complex. Structure AREHOU93 (Figure

overlap geometry, we calculated the energy of the interaction for model system with two (2,2′-bipyridyl)dicyanonickel(II) complexes (Figure 6) made from this structure, AREHOU, by substituting thiocyanato and isothiocyanato ligands with cyano ligands and by substituting metal ion with nickel(II). The calculated energy of stacking interaction in this model system is −31.66 kcal/mol (Table 1). The group of structures with overlap type Ib, with rMM of fset values between 3.0 and 5.0 Å and the value of angle φ lower than 40°, is the most numerous and includes 82 interactions (Table 1). Bipy ligands overlap in such way that there are chelate−chelate, pyridine−chelate, and pyridine−pyridine overlaps (Figure 7). The metal ions do not overlap with bipy ligand from the other complex. Visual inspection of the structures shows that in most of them ligands at third and fourth coordination position are planar or not voluminous, and bipy

Table 1. Number of Stacking Interactions, Geometric Data,a and Energies of Interactions group of structures

Nb

T1 (deg)

I

170

0−10

0−10

II

94

0−10

170−180

T2 (deg)

overlap type

Nb

limiting parameters

Examplec

ΔEd (kcal/mol)

Ia Ib Ic Id IIa IIb

14 82 46 28 36 58

rMM < 3.0 Å, φ < 40° 3.0 Å < rMM < 5.0 Å, φ < 40° 5.0 Å < rMM < 6.0 Å, φ < 30° 6.0 Å < rMM < 8.0 Å, φ < 30° rMM < 7.0 Å, φ < 40° (rMM < 7.0 Å, φ > 40°) + (rMM > 7.0 Å)

AREHOU GETMIC GETMIC NEZPIR LASXUY IROLOQ

−31.66 −26.11 −16.46 −12.65 −16.13 −7.26

a

Geometric parameters are given in Figure 1. bNumber of stacking interactions. cRefcodes of crystal structures selected as examples of overlap types (Figures 6, 7, 8, 9, 12, and 13). dInteraction energies at TPSS-D3/def2-TZVP level were calculated for model systems made from crystal structures selected as examples of overlap types (Figures 6, 7, 8, 9, 12, and 13). 3883



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ligand with the other ligands (H1···OX = 2.81 Å, H2···OY = 3.20 Å and H3···OY = 2.60 Å). The calculated energy of stacking interaction between two model systems of bipy complexes (Figure 8) is −16.46 kcal/mol (Table 1). The group of structures with overlap type Id includes 28 interactions. In this group of structures, with rMM of fset values larger than 6.0 Å, the value of angle φ is less than 20° (Table 1), and only pyridine rings are involved in overlap (Figure 9). Figure 7. Perspective view of crystal fragment of GETMIC (bis(acetamidato-N)-(2,2′-bipyridyl-N,N′)-platinum(II) hydrate), as a selected example for structures of type Ib and view of model system of (2,2′-bipyridyl)dicyanonickel(II) complexes, made from this crystal structure.

ligands interact with ion (or molecule) from external coordination sphere of the complex, which is located above other ligands, in the vicinity of the metal ion. Structure GETMIC94 (Figure 7), which represents bis(acetamidato-N)(2,2′-bipyridyl-N,N′)-platinum(II) hydrate, is an illustration of this overlap. In this structure, bipy ligands, besides the stacking, form simultaneous C−H/O intermolecular interactions with surrounding water molecules (H1···OX = 2.95 Å, H2···OY = 3.10 Å, and H3···OY = 2.52 Å). The water molecules are located above other ligands and form additional hydrogen bonds as H acceptors (H4···OX = 2.46 Å and H5···OY = 2.44 Å). For this overlap geometry we also calculated energy of the interaction for model system (2,2′-bipyridyl)dicyanonickel(II) complexes (Figure 7). Calculated energy of stacking interaction between two (2,2′-bipyridyl)dicyanonickel(II) complexes is −26.11 kcal/mol (Table 1). The calculated energy shows that interaction with this overlap (Figure 7) is weaker than with overlap presented in Figure 6. This can be anticipated based on the overlap area; overlap area in Ia (Figure 6) is larger than in Ib (Figure 7). In type Ic, rMM offset values are larger than in type Ib (above 5.0 Å and below 6.0 Å), and values of angle φ are below 20° (Figure 5). The group Ic is smaller group than Ib; it includes 46 interactions (Table 1). In these structures pyridine−pyridine and chelate−pyridine overlaps can occur, while chelate−chelate overlap does not occur (Figure 8). The other two ligands are

Figure 9. Perspective view of crystal fragment of NEZPIR ((2,2′bipyridyl)-bis(2-hydroxy-3-(3-methyl-2-butenyl)-1,4-naphthoquinone)-copper(II)), as a selected example for the structures of type Id, and view of model system of (2,2′-bipyridyl)dicyanonickel(II) complexes, made from this crystal structure.

In this overlap type bipy ligand interacts with the ion or molecule which is located above the metal ion. An example of this overlap is structure of (2,2′-bipyridyl)-bis(2-hydroxy-3-(3methyl-2-butenyl)-1,4-naphthoquinone)-copper(II) (structure NEZPIR95), shown in Figure 9. In this structure bipy ligand interacts with oxygen atom of 2-hydroxy-3-(3-methyl-2butenyl)-1,4-naphthoquinone ligand (H1···O = 2.48 Å and H2···O = 2.61 Å), pseudocoordinated to Cu2+ ion (Cu2+···O = 2.40 Å). The calculated energy of stacking interaction for model system (Figure 9) is −12.65 kcal/mol (Table 1). The second group of structures (II) with torsion angle T1 values close to 0° and T2 values close to 180° is smaller than group I, and it includes 94 interactions (Table 1). In group II, bipy ligands only partially overlap, and this partial overlapping always includes at least one pyridine−pyridine overlap. The histograms showing the distribution of the rMM and R values for structures in group II are presented in Figure 10. For a substantial number of interactions, the rMM is in the range of 5.0−10.0 Å, and the distribution of the offset values shows a maximum between 6.0 and 7.0 Å (Figure 10). The rMM values are higher than in group I (Figure 5) as a consequence of the smaller overlap area. The normal distances (R) in group II have lower values (Figure 10) than in group I (Figure 4). This could be the result of higher repulsion between bipy ligands, caused by larger overlapping surface in the interactions of the group I. The plot of angle φ versus the offset rMM for group II is shown in Figure 11. The overlap manner in this group is not a unique one, as indicated by the wide range of rMM and φ values. In some structures, chelate−pyridine overlaps also exist; however, in group II there are no chelate−chelate overlaps. The pyridine−chelate overlaps occur in contacts with rMM values lower than 7.0 Å and the values of angle φ lower than 20° (IIa) (Table 1), while in the other contacts of group II (IIb) there are only pyridine−pyridine overlaps. In the structures of group II at least one of the other ligands besides bipy is usually voluminous, and stacking interaction between two complexes can be additionally favored by the interaction of bipy ligand of one complex with the other ligand

Figure 8. Perspective view of crystal fragment of GETMIC (bis(acetamidato-N)-(2,2′-bipyridyl-N,N′)-platinum(II) hydrate) as a selected example for the structures of type Ic and view of model system of (2,2′-bipyridyl)dicyanonickel(II) complexes, made from this crystal structure.

not planar, and stacking interaction is very often additionally stabilized by the interaction of C−H group of the bipy ligand in the first complex with other ligand of the second complex. In crystal structure GETMIC, stacking interactions form chains, where Ib and Ic types of overlap alternately appear in the chains. Ic type of overlap (Figure 8) coexists with interaction of bipy 3884



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Figure 10. Distributions of rMM parameter and normal distance R (Å) for the interactions in group II.

which is located above one of the other ligands (H3···O2 = 1.74 Å) and with the fluoro atom (H4···F = 2.58 Å) from the voluminous second ligand. The calculated energy of stacking interaction between two (2,2′-bipyridyl)dicyanonickel(II) complexes in model systems made from LASXUY structure (Figure 12) is −16.13 kcal/mol (Table 1). In the structures that belong to IIb overlap type, only one pyridine fragment is involved in overlap and ion or molecule from the external sphere of the complex packs above bipy ring. This type was noticed in 58 interactions. An example of IIb overlap is the crystal structure IROLOQ97 (bis(2-aminopyridine)-(2,2′-bipyridine)-platinum(II) tetracyanoplatinate(II) dihydrate) (Figure 13).

Figure 11. Plot of angle φ versus of fset rMM, for interactions in group II.

in the second complex. This manner of overlap is very often a consequence of steric overlaid by other species, which interacts with the π-system of the bipy ligand. The type IIa overlap was found in 36 interactions. These are interactions where one of pyridine rings of the first ligand overlaps with pyridine and chelate ring of the second complex, simultaneously. Chelate−chelate overlap does not occur, and metal ion is not involved in overlap with the bipy ligand. An example of such overlap is the crystal structure LASXUY96 representing (2,2′-bipyridyl)-(1,1,1,3,3,3-hexafluoro-2-propanolato)-phenolato-palladium(II) phenol solvate (Figure 12). In this structure bipy ligand interacts with oxygen atom of phenol molecule (H1···O1 = 2.45 Å and H2···O1 = 2.55 Å),

Figure 13. Perspective view of crystal fragment of IROLOQ (bis(2aminopyridine)-(2,2′-bipyridine)-platinum(II) tetracyanoplatinate(II) dehydrate), as a selected example for the structures of type IIb, and view on model system of (2,2′-bipyridyl)dicyanonickel(II) complexes, made from this crystal structure.

In this structure the bipy ligand interacts with tetracyanoplatinate(II), forming two C−N/H interactions (H1···N1 = 2.64 Å and H2···N2 = 2.84 Å). This Pt complex is located above second bipy ligand and simultaneously interacts with other ligands of the second bipy complex (H3···N2 = 2.36 Å and H4···N2 = 2.53 Å). The calculated energy of stacking interaction in model systems made from IROLOQ97 structure (Figure 13) is −7.26 kcal/mol (Table 1). The interaction energy indicates that the interaction in IIb type is the weakest of all overlap types, as can be anticipated based on the small overlap area. The results obtained by calculations show that the most stable model systems have the largest overlap surface, with ligands at third and fourth positions included in the overlap (Ia overlap type). Although the calculated interaction of Ia type is the strongest, Ia type overlaps are not the most numerous in the crystal structures. Namely, model systems used for the calculations have small ligands at third and fourth positions

Figure 12. Perspective view of crystal fragment of LASXUY ((2,2′bipyridyl)-(1,1,1,3,3,3-hexafluoro-2-propanolato)-phenolatopalladium(II) phenol solvate), as a selected example for the structures of type IIa, and view on model system of (2,2′-bipyridyl)dicyanonickel(II) complexes, made from this crystal structure. 3885



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((2,2′-bipyridyl)-5-nickela-3,3,7,7-tetramethyl-trans-tricyclo[4.1.0.02,4]heptane).

(CN groups), while in most of the crystal structures other ligands coordinated to the metal are voluminous; hence, the overlap type Ia is not possible. This is the reason why the structures with overlap type Ib, Ic, and Id, in which the ligands at third and fourth coordination positions are not included in interaction, are more numerous than those with overlap type Ia (14 interactions) (Table 1). The number of structures with overlap type Ib (82 interactions), Ic (46 interactions), and Id (28 interactions) can be correlated with the results of calculations (Table 1), which show that the reduction in the overlap area leads to a decrease of the interaction energy. The weakest interaction, with an energy of −7.26 kcal/mol, is calculated in model system of overlap type IIb where only one pair of pyridyl rings overlap. The interaction in model system of overlap type IIa is remarkably stronger, with an energy of −16.13 kcal/mol; however, the data from the CSD showed that the contacts with overlap type IIb (58 interactions) are more numerous than the contacts with overlap type IIa (36 interactions). The large number of structures with weaker interaction can be the consequence of additional interactions in supramolecular structures in crystals. Namely, in type IIb, a large area of the bipy ligand is available for additional contacts; in structure IROLOQ97 (Figure 13) the bipy ligand forms interaction with the bipy ligand from the other complex and simultaneously forms interaction with the tetracyanoplatinate(II) complex. The loss of energy in structures with overlap type IIb is compensated by the additional stabilization in the supramolecular structure due to simultaneous interactions. Comparison of the interaction energy for overlap type Id (−12.65 kcal/mol), in which there are two pairs of pyridine− pyridine overlap, with the energy of interaction for overlap type IIa (−16.13 kcal/mol), in which there are two pairs of chelate− pyridine overlap, indicates that chelate−aryl stacking interactions are stronger than aryl−aryl stacking interactions. It is in accordance with findings that chelate−benzene stacking interactions are stronger than benzene−benzene stacking interactions.58,91 It is also interesting to compare interaction energy for overlap type IIb (−7.26 kcal/mol), in which only one pair of pyridyl rings overlap, with energy of stacking interactions between pyridine molecules (−4.08 kcal/mol).33 This indicates that stacking interactions become more favorable when pyridyl rings are coordinated to a metal, which is in accordance with Hunter−Sanders rules.98 Packing in Crystal Structures. Analysis of packing in crystal structures showed that stacking interactions form stacking chains and dimers. Stacking chains are formed only by overlap of bipy ligands. We analyzed packing in all structures with all values of torsion angles T1 and T2, not only structures belonging to group I and II. In 64% of total interactions, stacked bipy ligands form chains, while in 31% of interactions stacked bipy ligands form dimers. In some chains the same overlap is permanently repeated (15%), while in most of the structures two types of overlap alternately appear in the chains (49%). Stacking chains with the repeating of the same overlap appear in structures where there is no ion or molecule from the external sphere. Most of the contacts have no voluminous or planar other ligands. If the other ligands are voluminous, their configuration is the same on both sides of the mean plane of the bipy ring. An example of the chains with the same overlap is the structure PNIHEP99 ((2,2′-bipyridyl)-5-nickel-3,3,7,7-tetramethyl-trans-tricyclo(4.1.0.02,4) heptane). In this structure (Figure 14), the other ligand is voluminous chelate ring

Figure 14. Perspective view of crystal fragment of PNIHEP ((2,2′bipyridyl)-5-nickela-3,3,7,7-tetramethyl-trans-tricyclo(4.1.0.02,4)heptane), as an example of stacking chains with repeating the same overlap; in this case the other ligand is voluminous.

Stacking dimers were found in 31% of interactions. The reasons for forming dimers instead of chains in crystal packing can be voluminosity of the other ligand, but also the possibility of either other ligand or molecule (ion) from the external sphere of the complex to interact with the π-system of the bipy ligand. Example of a dimer is the crystal structure IVUFAG,100 shown in Figure 15. In this structure, both methyl groups of 2,5-bis(dimethoxymethyl)phenyl ligand interact with the πsystem of the bipy ligand as hydrogen atom donors. However, in 12% of total interactions the other ligand contains an aromatic ring, which forms π−π stacking interaction with the

Figure 15. Perspective view of the crystal fragment of IVUFAG (chloro-(2,2′-bipyridine)-(2,5-bis(dimethoxymethyl)phenyl)palladium(II)), as an example of stacking dimer. 3886



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bipy ligand. In this way stacking chains are formed, and stacking interactions of two different sorts of molecules alternately appear in the chains. An example of such packing is the crystal structure EVEWEH,101 shown in Figure 16. In this case, the aryl group of the 1,2-benzenediselenolato ligand forms π−π stacking interaction with the bipy ligand of the neighboring complex.

Figure 16. Perspective view of crystal fragment of EVEWEH ((2,2′bipyridine-N,N′)-(1,2-benzenediselenolato-Se,Se′)-platinum dichloromethane solvate), as an example of stacking dimer.

Figure 17. Four geometries of [Ni(CN)2bipy]2 dimer used for calculations; the geometric parameters and interaction energies are shown. Calculations were done on the TPSS-D3/def2-TZVP level of theory, with effective core potential used for metals (SDD).

In our previous work,63,64 it was shown that for the terpy and phen complexes stacking chains with alternating overlaps are the predominant type of packing; the chains occur approximately three times more often than dimers. Data in this work indicate that bipy complexes also show preference for stacking chains. In bipy complexes, the chains occur nearly two times more frequently than dimers. Since terpy and phen ligands coordinated to metal ions form planar systems of five and four rings, respectively, the explanation for a larger number of chains in the case of terpy and phen complexes can be connected to the number of rings that has propensity to form stacking interactions. It is obvious that a decrease of the number of rings leads to a reduction of the stacking chains fraction in crystal structures. The probable reasons for these may be that interactions of ligands with the environment are stronger than stacking interactions. Relationship of Calculated Interaction Energy and Overlap Surface. Analysis of the data obtained from CSD showed several types of overlap patterns between bipy complexes. In previous sections we presented the data on the interaction energies of all overlap types. In order to find relationship between interaction energy and overlap surface of bipy complexes, the calculations on several geometries of stacked [Ni(CN)2(bipy)] dimers were performed. In all model systems Ni complexes have head-to-tail orientations, which is the most common orientation in crystal structures. Several geometries with different overlap surfaces are shown in Figure 17. By increasing rMM, from geometry a to geometry c, the overlap surface decreases. In geometry d we changed both distance rMM and angle φ, in the way that the distance between centers of interacting pyridine rings is the same as in geometries a and c; offset for two rings is 1.5 Å. The values of optimal normal distance R for geometries in Figure 17 were obtained by single-point energy calculations by changing the normal distance R, while the geometries of the monomers were kept rigid. The calculated interaction energies and geometric data are presented in Figure 17. The interaction energy decreases with decreasing overlap surface. The strongest interaction is in geometry a (−27.7 kcal/ mol), with the largest overlap surface, while the weakest interaction is in geometry c (−12.5 kcal/mol), with the smallest

overlap surface. Geometry d has a somewhat smaller overlap surface than geometry b and somewhat smaller interaction energy (Figure 17). Previous data on stacking interactions in benzene32 and pyridine dimers33 show that displaced geometries are more stable than face-to-face geometries; hence, geometries with a larger overlap surface do not have stronger interaction energies. On the other hand, data in this work on bipy complexes show that decreasing overlap surface decreases the strength of the stacking interaction (Figure 17 and Table 1). This difference between stacking interaction of benzene and pyridine and interactions of bipy metal complexes is probably caused by the number of nitrogen atoms and metal atoms in bipy complexes. Namely, nitrogen atoms and metal atoms in complexes make electrostatic interactions more favorable in geometries with large overlap surface. It is also interesting to compare interaction energy in the model system in Figure 13, where only two pyridine rings overlap, with the interaction energy of two uncoordinated pyridines.33 The interaction energy in the model system in Figure 13 is remarkably stronger (−7.26 kcal/ mol, Table 1) than the interaction energy between two uncoordinated pyridines (−4.08 kcal/mol). Hence, coordination of pyridine influences stacking interaction energies significantly.

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CONCLUSION Geometries of stacking interactions between bipyridine squareplanar complexes in crystal structures were analyzed, using the data obtained from the CSD. Geometric analysis showed that two stacked complexes are head-to-tail oriented in most of the interactions, with a large portion of bipy ligands involved in the overlap. We used geometric parameters, torsion angle T2, distance rMM, and angle φ, to classify mutual bipy ligand overlaps into six types (Ia, Ib, Ic, Id, IIa, and IIb type). The most numerous are the structures with overlap type Ib, with quite a large overlap surface (rMM ≤ 5 Å, φ ≤ 35°): there are pyridine− pyridine, chelate−chelate, and pyridine-chelate overlaps. The geometry of the interaction is very often influenced by two ligands coordinated at the third and fourth coordinating 3887



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positions or by molecules (ions) from the environment in crystal structures. Analysis of packing in crystal structures showed that stacked bipyridines mainly form stacking chains, and the preference for stacking chains is nearly two times larger than for dimers. In most of the structures with chains, two different types of overlap alternately appear in the chains. The results of calculations show that the energies of stacking interactions between two [Ni(CN)2bipy] complexes can be very strong; the strongest calculated interaction has the energy of −31.66 kcal/mol. The weakest calculated interaction, in which only one pair of pyridyl rings overlap, has the energy of −7.26 kcal/mol. As it might be anticipated, the results of calculations show that the reduction in the overlap surface leads to a decrease of interaction energies. The results also indicate that the interaction of pyridine rings coordinated to the metal (−7.26 kcal/mol) is remarkably stronger than stacking interaction of uncoordinated pyridines (−4.08 kcal/mol). The results presenting geometries of stacking interactions in crystal structures and very strong interaction energies of squareplanar bipy complexes can be very important for different molecular systems where bipy complexes can be used.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Telephone: +381-11-3336-605. Fax: +381-11-638-785. Author Contributions #

All authors contributed equally.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the Serbian Ministry of Education, Science and Technological Development (Grant 172065). ABBREVIATIONS CSD, Cambridge Structural database; DFT, density functional theory; terpy, terpyridyl; phen, phenantroline; bipy, bipyridine; DSC, dye-sensitized solar cells; BSSE, basis set superposition error

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Stacking Interactions between Square-Planar Metal Complexes with 2

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